Ultrasonic sensors are utilized in diverse equipment including medical ultrasonic wave echo diagnostic equipment and ultrasonic flaw detection equipment for non-destructive testing.
Up until now ultrasonic sensors mainly utilized an oscillating piezoelectric element but progress has been made in developing capacitive-detection type ultrasonic sensors using MEMS technology that has made significant advances in recent years.
This capacitive detection type ultrasonic sensor utilizes an oscillator made up of a cavity between mutually opposing electrodes formed on a semiconductor substrate. Applying overlapping (multiplexing) direct current and alternating current voltages to each electrode causes a membrane to oscillate in the vicinity of the resonant frequency to generate ultrasonic waves. Current research efforts are attempting to apply this principle to develop electrode structures to contrive 1.5 dimensional arrays for short axial variable focusing and two-dimensional arrays for real-time 3D imaging.
Technology for this type of ultrasonic sensor is disclosed for example in U.S. Pat. No. 6,320,239B1. This specification discloses technology for a capacitive detection type ultrasonic transducer utilizing a lower electrode on the silicon substrate.
Technology was also disclosed for a capacitive detection type ultrasonic transducer formed above the patterned lower electrode (See for example, U.S. Pat. No. 6,271,620B1, “2003 IEEE Ultrasonics Symposium” (USA), 2003, p. 577-580).
Moreover, U.S. Pat. No. 6,571,445B2 and U.S. Pat. No. 6,562,650B2 disclose technology for a capacitive detection type ultrasonic transducer on the upper layer of a signal processor circuit formed on a silicon substrate.